This invention relates to franking machines and in particular to such machines provided with franking printing means controlled by electronic accounting and control means to maintain an accounting record of data relating to franking impressions which have been printed.
Known franking machines utilise a mechanically operated print drum to print franking impressions. The print drum carries settable print wheels for printing the value of franking and the date of franking. The setting of the print wheels, and hence the value of the franking impression printed, is controlled by a franking meter which includes means for carrying out accounting functions in respect of usage of the meter in franking operations. Usually the meter includes a descending register which is reset to record a value of credit available for use in franking and in each franking operation this register is decremented by the value of franking impression printed. The accumulated value of franking used in franking operations is registered in an ascending register, this register being incremented in each franking operation by the value of franking printed Mechanical interlocks are provided to prevent fraudulent attempts to operate the printing mechanism independently of control by the meter and to prevent fraudulent attempts to reset the print wheels to print a franking value different from that registered by the meter.
With the availability of electronically operated printing devices such as thermal print heads and ink jet print heads, it is desirable to replace the mechanical drum print mechanism with electronically operated print device in order to provide greater flexibility in printing and to avoid the need for complex and expensive electro-mechanical interfaces between the electronic accounting and control circuits and the mechanical print elements of the drum printer. However such electronically operated printing devices could be operated by the external application of electrical signals to the print head elements or the drive circuits thereof and hence may be capable of being operated in a fraudulent manner. Accordingly measures must be taken to ensure security of the printing of franking impressions and the accounting thereof. One method of overcoming this problem is to house not only the meter circuits but also the printer, the drive circuits therefore and the interconnections between the meter and the printer in a secure housing. This has the disadvantage that, if a fault arises in the printer, repair of the fault requires access to the secure housing and as a result the postal authority may require the integrity of the accounting records maintained in the meter to be verified prior to re-use of the machine. It would be advantageous to place within the secure housing only those circuits concerned with performing functions for which there is a need for security and to house other elements of the franking machine externally of the secure housing thereby reducing the occasions when there is a need for access to the secure housing and to take measures to ensure that elements of the franking machine, such as the printer, housed externally of the secure housing cannot be operated in a manner to cause printing of a fraudulent franking.